Guest Post: Burning Buried Sunshine

Solar energy is often criticized for its inefficiency – that only about 10% of the sunlight that hits a common commercial solar panel will be converted into electricity. Similar criticisms are voiced against biofuels, which have a solar energy to biofuel conversion efficiency of less than about 2%.* But how do these efficiencies compare to other sources of energy, like oil? Turns out – solar and biofuels do pretty well.

First consider this; we truly have three primary sources of energy: nuclear, geothermal, and solar**. Solar energy is the original source of energy for wind, biomass, fossil fuels, and even hydroelectric. The sun creates temperature gradients on the surface of the Earth, which creates wind. Biomass photosynthesis is powered by solar photons, and sometimes that biomass falls to the ground, gets buried, and cooked into fossil fuels. Hydroelectric dams harness the potential energy of water, but the water has to get “uphill” somehow. This happens when the sun lifts the water through evaporation, which is later released as rain.

These ‘sources’ of energy - wind, biomass, and hydro - are not true sources; they are really just different ways to carry, or convert solar energy. When comparing these types of energies, it is useful to think about how efficiently each type converts solar energy into a useful form of energy. In the cases of wind, hydro and solar, the useable form of energy is usually electricity; with biomass and oil, we get some sort of liquid fuel that can be used to either move your car or generate electricity. For the sake of simplicity, we will say that all of these sources end up as electricity.

Now we know that solar panel conversion efficiency is about 10%, and the biomass solar conversion efficiency is about 2%. As it turns out, that’s pretty good.

Let’s look at fossil fuels, oil in particular. Now, fossil fuels are pretty great. They are very energy dense, and have enabled society to do some amazing things. If you think of fossil fuels as a source of energy, they are efficient. But, fossil fuels are actually a secondary form of energy, and when you consider how conventional fossil fuels are naturally made, the solar conversion efficiency is astonishingly low.

How low? Let’s have a look at how oil is made, and how efficient each step is:

First, you need sunlight to grow plants, but photosynthesis is only about 1.7% efficient.

Only about 2% of the biomass that grows is actually preserved, and ends up deeper in the Earth.

About 74% of the sequestered biomass turns into oil.

Only about 2.8% of that oil gets trapped.

Only about 25% of the trapped oil is actually recoverable by humans.

About 90% of the recovered oil goes to products, since about 10% has to be fueled (usually as natural gas) to run the extraction and refining process.

Finally, internal combustion engines are only about 20% efficient.

So if you total that all up, fossil fuels are about 0.000003% efficient at converting sunlight to energy. Put another way: If you want one Watt of energy from solar panels, you need about 10 Watts of sunlight. If you want one Watt of energy from biomass, you need about 50 Watts of sunlight. If you want one Watt of energy from oil, you need over three million Watts of sunlight.

The actual number for fossil fuels is 3.15 X 10^6 Watts of sunlight per watt of fossil fueled energy. For you math lovers, that is close to pi million watts of sunlight.

We can also think about this is terms of land area required. If you want to run your house (about 4 kilowatts) for a year, you need about 35,000 kilowatt-hours of electricity. If you want to get that energy from solar, you will need about 10 acres of solar panels. To get that energy from biofuels, you will need about 50 acres of farmland. But to get that same amount of energy from fossil fuels, you need the sun to shine on a swath of land about the size of Connecticut, for an entire year, for one house.

That means that if all the land in the United States was devoted to producing energy through continuous sustainable fossil fuel production using the sun and the natural conversion process, the entire United States could only power about 600 homes. But unfortunately for those 600 homes, they would have to wait a couple million years before their energy was delivered.

Fortunately, we currently don’t have to rely on the Earth’s natural fossil fuel formation processes. We have an abundance of oil stored in the crust, and if and when that runs out, we have more efficient ways of making synthetic fossil fuels using biomass than the way the Earth does it naturally. So oil is great, for now, but as it turns out, solar and biomass are much more efficient in the long run.

*The advantage of biofuels should be noted though – that the collector assembles itself. You don’t have to build anything, you basically just throw a seed on the ground and your solar collector grows out of thin air. With solar panels, you have to actually build and transport the collector, which is annoying.

**There may be one tiny exception, and that is tidal power, which is powered by the gravitational forces of both the sun and the moon.

Author's note: The inspiration for this article came from the Fundamentals of Renewable Energy Processes Class at Stanford University, taught by Dr. Adam Brandt. His inspiration for this calculation came from Dr. Jeffery Duke’s publication, Burning Buried Sunshine: Human Consumption of Ancient Solar Energy.

About the Author:

Scott McNally is an energy engineer who has spent the past year working on national energy policy issues in Washington, DC. He has worked as an ORISE Fellow with the Department of Energy’s ARPA-E Program and an energy and climate researcher with the White House Council on Environmental Quality. Previously, Scott was a project engineer for Shell Oil Company and an environmental engineer for Valero. Scott has a B.S. in Chemical Engineering from the University of Texas at Austin, and is currently completing a Masters in Energy Resources Engineering at Stanford University. Scott was invited to be a guest blogger by Plugged In’s Melissa C. Lott. You can reach Scott via e-mail at scottmcnally at gmail dot com.

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